Fuel-metal compatibility test unit

ABSTRACT

A METHOD AND TEST APPARATUS IS PRESENTED WHICH MEASURES GAS PRODUCED BY CATALYTIC DECOMPOSITION OR CHEMICAL REACTION OF A HYDRAZINE-TYPE FUEL AND A METAL SPECIMEN.

United States 3,744,971 Patented July 10, 1973 US. Cl. 23-230 R 7 ClaimsABSTRACT OF THE DISCLOSURE A method and test apparatus is presentedwhich measures gas produced by catalytic decomposition or chemicalreaction of a hydrazineatype fuel and a metal spec1men.

The invention described herein may be manufactured, used and licensed byor for the Government for governmental purposes without the payment tous of any royalties thereon.

The development of high strength/low weight materials, such as nickelmaraging steel, cryogenically formed stainless steel, and titaniumalloys have provided designers with structural materials possessingyield strengths of up to 400,000 p.s.i. Fabrication of tankage fromthese materials results in higher ratios of payload to total weight ofmissile (or a greater range and shorter time-to-target) which is thegoal of rocket technology.

A lack of compatibility, however, of some of these materials withcertain liquid propellants has restricted their use as tankage. Onecorrective approach has been to provide an internal metallic protectivecoating which neither reacts with nor catalyzes a decomposition of theparticular liquid propellant. Such a coated material may be said to becompatible with the propellant if it fulfills the above requirements.Since a five year storage life is desired in prepackaged liquid rockets,it is clear that a method and apparatus to predict long term gaseousbuildup from propellant decomposition-reaction is needed. Such exposuretests must be at least of one year duration and at an elevatedtemperature (to accelerate any chemical or catalytic reactions) to bemeaningful.

It is therefore an object of this invention to provide a method ofdetermining gaseous buildup from liquid propellants induced eithercatalytically or through chemical reaction with a high strength materialtankage.

Yet another object of this invention is to provide an apparatus tomeasure the above mentioned gaseous buildup.

A still further object of this invention is to provide an apparatussuperior to previous units in the quantitative containment of thegaseous vapors released by the propellant fuel.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same become better understood byreference to the following description.

This invention relates to a method of determining the rate ofdecomposition of a liquid fuel, preferably a hydrazine-type fuel, andpresents an apparatus for the quantitative measurement of gases evolvedfrom the fuel caused by catalytic decomposition or reaction with highstrength low weight metals used as tankage with said fuels.

DESCRIPTION OF THE APPARATUS A preferred embodiment of the apparatusconsists of a heat resistant glass unit incorporating a chamber 1 forholding the fuel 2, for example a hydrazine-type fuel and metal specimen3 and an attached calibrated (centimeter scale) mercury-filled manometer4 to measure any gas released by catalytic decomposition or by reactionof the specimen whose compatibility with the preferred hydrazine fuel isbeing tested. The various legs of the manometer and fuel test bulb arestrengthened with glass braces 5. An enlarged section of the manometernear the fuel bulb allows for a reservoir of mercury to increase thetime interval between adjustments of the level of mercury in themanometer due to release of gas by any reaction during the test and alsopermits use of a shallow temperature bath (necessary to acceleratedecomposition or reaction in simulated storage testing) by reducing thelength of the manometer section of the unit.

Initially the ends of the specimen chamber are left open to load thespecimen and the fuel and then are fusionsealed to contain the gasvapors. All areas of the unit contacted by the fuel or fuel vapors arearranged to be below the surface of the constant temperature bath toreduce transfer of the fuel to the manometer side of the unit bycondensation.

A porous frit 6 with holes too small to permit the passage of mercurybetween the test chamber and the manometer is installed in a connectingtube 7 which is sloped toward the test chamber to return any fuelcondensate to the chamber. The apparatus has a centimeter scale attachedto the manometer tube which allows reading of the height of the mercurycolumn indicating changes in the pressure and volume of the gas evolvedfrom the propellant.

Such a fusion closed system, as above, was arrived at when it was foundthat closure seals (a stop cock type system) employing closure greasessuch as, vaseline, silicone grease, vacuum pump oil or Teflon(tetrafluoroethylene) grease, were inadequate. Leakage problems wereencountered due to the greases solubility and reactivity with thehydrazine fuel vapors and secondly the high temperature F.) at which theexperiment was carried out.

DESCRIPTION OF THE TEST METHOD The glass apparatus was checked with apreloading leak-test procedure which consisted of two tests.

The unit was cleaned by the following steps: (a) Submerge and soak in anitric acid solution (1:1 dilution of conc. (HNO for two hours. (b)Rinse with distilled water. (c) Soak two hours in aqueous NH (1 partcone. NH, plus 4 parts H 0). (d) Rinse with distilled water. (e) Ovendry.

The unit was then clamped in an inverted position. The specimen to betested (previously cleaned by conventional pre-plating proceures,followed by a soak in the above NH solution, rinsed, and air-dried) wasinserted into the specimen chamber through its open end. The specimenchamber was then sealed by fusing and drawing the glass. Adequateprecautions were taken during this operation to avoid heating andoxidizing the specimen (step-wise drawing with intermittent cooling, airblast cooling of the outside of the opposite end of the specimenchamber, and simultaneous flushing of the specimen chamber withnitrogen). The unit was then pumped down to a low pressure through theopen end of the manometer tube and all seals were checked with aspark-type, high voltage leak tester. Leaks, if found, were repaired.For further leak-testing, mercury was then added to the manometer andthe unit was pressurized with N using a 2 mm. Teflon tube pushed throughthe mercury column, to the limit of the manometer tube, thus exposingall fusion seals and joints to the gas.

After about two weeks, if no evidence of a leak was observed, the unitswere then opened by breaking the seal at the top of the specimen chamberand enough fuel was added to cover the specimen.

A plastic syringe with its needle attached to a 2 mm.- dia. Teflon tubewas used to introduce the fuel into the lower end of the specimen bulbwithout Wetting the wall of the inlet tube. A temporary closure of theinlet tube was then made with a Teflon plug. The fuel was left in theunit about one week to leach possible residual impurities from thespecimen and the interior of the unit. This fuel was then removed by useof the syringe and replaced with fresh fuel, again avoiding wetting ofthe upper end of the inlet tube. If wetted at this point, it wassometimes diflicult subsequently to obtain a leak-freefusion seal.

The fuel inlet at the top of the specimen chamber was then rescaled byfusion. The seal was inspected under magnification. If perfect, the unitwas flushed (alternately pressurized and depressurized) with N byinserting a long, 2 mm.-dia. Teflon tube through the open end of themanometer, through the mercury, and into the gas chamber above themercury in the manometer bulb. The unit was then placed in the constanttemperature bath to begin the test.

One specimen chosen was coated 18% nickel maraging steels (IS-18.5%nickel, 2-4.8% molybdenum, 0.2- 0.65% titanium, 0.1% aluminum, 0.03%maximum carbon, 7.8-8.7% cobalt, balance iron). Particular coatingsincluded aluminum, cadmium, nickel, 50/50 lead-tin solder, silver,chromium, gold, tin, zinc and cobalt.

Other specimens tested were molybdenum metal (99.9% molybdenum);cryogenically formed stainless steel 301 (17% chromium, 7% nickel, 0.15%maximum carbon, balance iron); AM 355 (15.5% chromium, 4.5% nickel, 3%molybdenum, balance iron); Inconel 718 (18.6% chromium, 18.5% iron, 0.4%aluminum, 0.9% titanium, 3.1% molybdenum, 0.04% carbon, 5% columbium,balance nickel); and Ti-6Al-4V (6.5% aluminum, 4% vanadium, 0.08%maximum carbon, 0.25% maximum iron, 0.05% nitrogen, 0.015% maximumhydrogen, balance titanium).

Fuels tested include hydrazine, monomethyl-hydrazine, unsymmetricaldimethyl hydrazine and mixtures of the above.

After approximately one year the change in volume and pressure due togas evolved by decomposition of the propellant was measured and thefollowing calculations made to determine the decomposition rate of theparticular specimen-fuel system.

The equation:

v.P.v.P. (1)

gives the amount of gas evolved, measured at atmospheric pressure (Pwhere V V and P P are initial and final total volume and pressurerespectively in the apparatus. The volume of the zero point of themanometer is determined from initial calibration of the apparatus. Fromthis value and the measured shift of the manometer level, V and V arecalculated. Height of the mercury in the manometer plus atmosphericpressure yield values of P and P The amount of gas evolved as calculatedfrom Equation 1 is due in part to catalytic decomposition at the surfaceof the metallic specimen, and in part from the inherent tendency of thepropellant to decompose at other locations, e.g., in the liquid itself,in the vapor phase, and at the glass propellant interface. The latter,referred to as background decomposition, is determined in an identicalapparatus, except that no specimen was present, and is subtracted fromthe volume of Equation 1 to obtain the actual amount of gas evolved dueto the specimen.

Since the catalytic decomposition rate of gas evolved due to contact ofa given specimen with the propellant is proportional to the area of thespecimen and to the time of contact, the reactivity might be expressedin terms of a single coefiicient defined as rate of gas evolved per unitarea per unit time.

As per Equation 1, AV is the total volume of gas evolved, measured at 1atmosphere pressure, that would change the volume and pressure in thesystem from V and P to V P If AV is the volume similarly determined dueto background condition, and d d are respective days of test, and A isthe area of the specimen, then R the rate coefficient, would be asfollows:

R is expressed as cmfi/day/cmfi.

For convenience, the rate of decomposition is also expressed in terms ofspecimen-tank pressure after one year, assuming a one cubic foot tankwith 10% ullage, and with the entire inner surface active. Pressure inthis assumed tank would be proportional to the rate coeflicient. Therelationships are indicated as follows:

V =ullage volume=10% of 1 ft. =2,832 cm. AV =volume of gas evolved in 1year in cm. measured at 1 atm., also y s) (R (365 days) (5,574 cm?) Pstandard atmospheric pressure, 14.7 p.s.i. P pressure developed in tankafter 1 year, p.s.i.

Applying the gas law and treating the specimen-tank as a closed system(fixed volume), the following equation can be obtained.

In Equation 4 P is absolute pressure. If P is designated as gagepressure (p.s.i.g.) after one year, then ipe s The above equations arederived directly from the simple gas law. Corrections might have to beapplied for errors due to gas law deviations, change of ullage owing toexpansion of liquid propellant with increase of temperature, etc., ifsuch errors are significant in a given system.

The following represents test data for specimens exposed to MHF-B at F.for various periods of time obtained by using the apparatus and methoddisclosed herein.

' A mixture of hydrazine and monomethyl hydrazine.

Calculated tank Time Rate pressure Area of under Gas eoefiieient afterone specimen test evolved b (cmfi/day/ year Specimen (0111. (days)(cmfi) cm?) (p.s.i.g.)

Aluminum l 12. 5 475 1. 1 0. 00019 2 Cadmium (thm)- 10.6 629 6.1 0. 000910 Cadmium- 14. 7 614 1. 1 0. 00012 1 Cadmium 1 H111) 14. 4 27 2. 3 0.0059 62 Electroless nickel 22. 2 788 17. 0 0. 00007 10 Electrolessnickel (Mk-bath) 13. 0 320 42. 0. 0125 132 Electroless nickel (EH4bath).-- 14. 2 390 113.0 0. 0203 214 Electroless nickel (0.1 mil) 15. 363 7. 5 0.0078 82 Electroless nickel (heated at Electroless nickel(oxidized). 14. 4 40 3.5 0. 0051 64 Electroplated nickel 14.0 211 19. 80.0067 71 Silver 14. 5 626 2.3 0. 00025 3 Molybdenum P 4. 8 368 217 0.123 1, 300

18% Maraging Steel 17.0 371 485 0. 097 1, 030

Based on a tank in the form of a cube, 1 it. volume, ullage.

Thusly through the practice of this invention, raw data may be obtainedwhich may be easily refined, by the provided mathematical calculation,into both rate coefficients and projected tank pressures for particularfuel-metal tankage systems.

It can be seen that the above described test device provides particularadvantage in the ready accessibility provided to the test sample. Themetal specimen 3 for example, may be removed from the test chamber 1 atthe completion of or during the test period. A full scale field test ofmetal-fuel compatibility requires relatively complicated removal andsectioning of the tankage material. Such sectioning is, of course, timeconsuming and expensive and also requires a termination of the testingof that particular tankagc-fuel system whereas the test samples whenpracticed with the present invention may be reinserted and furthertesting continued after preliminary examinations have been made.

Further, because the system is fusion sealed, inaccuracies due toleakage inherent in previous systems employing stopcocks and glass taperjoints are avoided.

We wish it to be understood that We do not desire to be limited to theexact details and compositions described in this specification forobvious modification will occur to a person skilled in the art.

We claim:

1. A method for determining the rate of catalytic decomposition andchemical reaction of a liquid hydrazine fuel in contact with a metalspecimen comprising the steps of:

contacting the metal specimen with the hydrazine fuel in a fusion sealedglass system consisting essentially of a glass chamber connected to amercury manometer; purging said system with an inert gas to displace theair and provide an inert atmosphere therein;

measuring the change in pressure and volume caused by the evolution ofgases from the catalytic decomposition and chemical reaction of thehydrazine fuel and metal specimen and the time period of such gasevolution;

calculating by use of simple gas laws the amount of gas evolved from theabove decomposition and reaction;

deducting any inherent catalytic decomposition of the fuel in theabsence of the metal specimen; calculating the decomposition rate of thefuel expressed as cm. of gas/cm. area of metal specimen/day.

2. The method of claim 1 wherein the test method is carried out at anelevated temperature.

3. The method of claim 1 wherein the fuel is selected from the groupconsisting of hydrazine, monomethyl hydrazine, unsymmetrical dimethylhydrazine, and mixtures thereof.

4. The method of claim 1 wherein the metal specimen is chosen from thegroup consisting of aluminum, 18% nickel maraging steel, molybdenummetal, cryogenically formed stainless steel, and coated 18% nickelmaraging steels.

5. The method of claim 4, wherein the metal specimen is coated 18%nickel maraging steel coated with a metal selected from the groupconsisting of cadmium, electroless nickel, aluminum, 50/50 lead-tinsolder, silver, chromium, gold, tin, zinc and cobalt.

6. An apparatus for measuring gases produced by the catalytic andchemical reaction of hydrazine-type fuels and metals comprising:

a fusion sealed glass system consisting essentially of a glass testchamber having an aperture therein for the escape of gases evolved; amercury filled calibrated glass manometer, open to the atmosphere,connected to said aperture; and

a gas permeable partitioning means located between the test chamber andthe glass manometer.

7. The apparatus in claim 6 wherein the gas permeable partitioning meansis a glass frit.

References Cited UNITED STATES PATENTS 12/1912 Bunzel 23-253 R 6/1967Scott 23-256 X OTHER REFERENCES L. F. Audrieth: The Chemistry ofHydrazine, page 102 1

